1. Field of the Invention
This invention relates to a gaseous fuel engine which runs on gaseous fuel such as hydrogen gas and a air-fuel ratio control system for the engine.
2. Description of the Prior Art
Hydrocarbon fuels such as gasoline which are liquid at ordinary temperatures have been wide used as automotive fuels. However gasoline engines and diesel-engines give off various air pollutants such as CO.sub.2, CO, HC, NOx and the like. Accordingly, as disclosed, for instance, in Japanese Unexamined Patent Publication No. 51(1976)-34308, there have been proposed gaseous fuel engines which run on hydrogen gas, methane gas, ethane gas or the like which produces no or a very small amount of CO.sub.2, CO or HC.
However since the density of gaseous fuel is very small as compared with that of liquid fuel such as gasoline, charging efficiency cannot be sufficiently high in the gaseous fuel engine and the engine output power cannot be sufficiently increased when the fuel is supplied to an intake passage where the fuel is mixed with intake air to form an air-fuel mixture and then the air-fuel mixture is introduced into a combustion chamber as in the conventional gasoline engines (pre-mixing fuel system).
Thus there has been proposed a gaseous fuel engine provided with a direct-injection fuel system in which the gaseous fuel is directly injected into the combustion chamber under pressure from the end of the intake stroke to the beginning of the compression stroke, i.e., after a sufficient amount of air is charged in the combustion chamber, thereby improving the charging efficiency and the engine output power. See Japanese Patent Publication Nos. 1(1989)-23659, 58(1983)-12458, and the like.
However such a direct-injection fuel system is disadvantageous in that the time for which the gaseous fuel is mixed with the intake air is very short and accordingly the thermal efficiency deteriorates and fuel economy lowers.
There has been proposed a gasoline engine in which a first fuel injection valve for injecting gasoline into intake air in the intake passage and a second fuel injection valve for injecting the same into the combustion chamber are provided and the first and second fuel injection valves are selectively used according to the operating condition of the engine. See, Japanese Unexamined Patent Publication Nos. 61(1986)-244821, 61(1986)-250364, 56(1981)-151213 and the like. That is, in the gasoline engine, the air-fuel mixture burns in a limited air-fuel ratio range and accordingly the air-fuel mixture is difficult to ignite during light load operation where the air-fuel ratio is set lean. Accordingly, gasoline is injected into the combustion chamber from the second injection valve during light load operation so that rich air-fuel mixture is locally formed around the spark plug (stratification), thereby improving the ignitability of the air-fuel mixture. On the other hand, during heavy load operation, gasoline is injected into intake air in the intake passage from the first injection valve to promote mixing of gasoline and air, thereby improving the thermal efficiency and the engine output power.
When the pre-mixing fuel system is used in the gaseous fuel engine, the air-fuel mixture burns substantially quickly under the normal condition, which makes higher the combustion temperature and results in increased amount of NOx. Accordingly, when the pre-mixing fuel system and the direct-injection fuel system are used together or selectively in the gaseous fuel engine, measure must be taken to prevent NOx from increasing or to reduce the amount of NOx produced. At present, however, there has been proposed no means which can effectively suppress the amount of NOx in the gaseous fuel engine where the pre-mixing fuel system and the direct-injection fuel system are used together or selectively.
Further the gaseous fuel engine involves a problem that since hydrogen is burnt, steam is formed in the cylinder and accordingly when the engine is cold, saturated steam in the burned gas is condensed and a drop of water adheres to the spark plug and/or the cylinder wall. The drop of water adhering to the spark plug can cause misfire and prevent the engine from being started. Further the drop of water adhering to the cylinder wall can flow into the oil pun to deteriorate the oil in the pun and to cause shortage of oil.
Though, as disclosed in Japanese Unexamined Patent Publication No. 2(1989)-267309, there has been proposed a hydrogen engine in which water component is removed from the oil, such approach adds to the cost, results in increase in the engine size, and at the same time can prevent water neither from adhering to the spark plug to cause the misfire nor from flowing into the oil pun.